Method and system for detecting operator alertness

ABSTRACT

A method and system for detecting operator alertness of an operator of a vehicle or machine comprises an image collection system. The image collection system collects reference motion data associated with an operator representation when the operator is in an alert state. An image processor determines observed motion data of one or more points of a three dimensional representation of the operator during a time interval An analyzer sends an alert signal to alert the operator if a detected angular shift of one or more points of the representation exceed a displacement threshold.

This document (Including the drawings) claims priority based on U.S.provisional Ser. No. 80/843,974, filed Sep. 12, 2006, and entitledMETHOD AND SYSTEM FOR DETECTING OPERATOR ALERTNESS, under 35 U.S.C.119(e).

FIELD OF THE INVENTION

This application relates to a method and system for detecting operatoralertness using stereo vision.

Monocular machine vision may offer limited capability in detectingoperator alertness. The position or movement of an operator may bedetected by analyzing a monocular image. However, monocular vision datamay be deficient in providing reliable determination of the position ormovement of an operator, which may require depth perception to ascertainthe three dimensional characteristics of the position or movement of anoperator. Operator alertness systems that rely on observation of theeyes of an operator encounter technical difficulties where the operatordoes not constantly face in a uniform direction (e.g., forward) or wherethe operator wears eyeglasses. Thus, there is need for a method andsystem for detecting operator alertness using stereo vision.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, a method and system fordetecting operator alertness of an operator of a vehicle or machinecomprises an image collection system. The image collection systemcollects reference position data or reference motion data associatedwith an operator, or a portion thereof when the operator is in an alertstate. An image processor determines observed position data or observed,motion data of one or more points of a three dimensional representationof the operator during a time interval. An analyzer sends an alertsignal to alert the operator if a detected angular shift of one or morereference points of the representation exceed a displacement threshold,or motion threshold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a block diagram of one embodiment of a system for detectingoperator alertness using stereo vision in accordance with the invention.

FIG. 1B is a block diagram of another embodiment of a system fordetecting operator alertness-using stereo vision in accordance with theinvention.

FIG. 2 is a flow chart that generally illustrates a first embodiment ofa method for detecting operator alertness.

FIG. 3 shows a diagram that illustrates the collection of stereo visiondata and determination of three-dimensional world coordinates for one ormore points associated with an operator, or portion thereof.

FIG. 4A shows a profile of an operator's head in a position thatindicates an alert state of an operator.

FIG. 4B and FIG. 4C show a profile of an operator's head in a positionthat indicates an inattentive state of an operator.

FIG. 5 is a flow chart of a second embodiment of a method for detectingthe alertness of an operator.

FIG. 6 is a flow chart of a third embodiment of a method for detectingthe alertness of an operator.

FIG. 7 is a flow chart of a fourth embodiment of a method for detectingthe alertness of an operator.

FIG. 8 is a flow chart of a fifth embodiment of a method for detectingthe alertness of an operator.

Like reference numbers in different drawings indicate like elements,steps or procedures.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with one embodiment, FIG. 1A illustrates a detectionsystem 11 for detecting an alertness or inattentiveness of an operator.As used herein, the operator generally refers to the operator of anyvehicle or any machine. The vehicle may comprise any train, bus,airplane, aircraft, helicopter, ship, boat, watercraft, automobile,truck, tractor, combine, agricultural equipment, construction equipmentforestry equipment, earth-moving machinery, mining equipment, or thelike. The vehicle may have, but need not have, unmanned,semi-autonomous, or autonomous capabilities with respect to guidance andnavigation. For example, the vehicle may have a guidance system thatuses a location-determining receiver (e.g., a Global Positioning Systemreceiver with differential correction) to determine a location of avehicle to facilitate-guidance of a vehicle in accordance with a pathplan.

The detection system 11 comprises an image collection system 15 coupledto an image processor 20. In turn, the image processor 20 is coupled toan analyzer 32. The analyzer 32 may provide a control signal to an alertdevice 40.

Image Collection System

The image collection system 15 comprises a left imaging unit 10 and aright Imaging unit 12 that are coupled to an image combiner 14 (e.g., aninterlacer). The image combiner 14 may be coupled to an interface 16.The interface 18 provides a communications interface (or a data buffer)for the stereo vision data transmitted from the image collection system15 to the image processor 20. An imaging system controller 18 maycontrol the optical characteristics of the left imaging unit 10, theright imaging unit 12, or the format of stereo vision data or image dataoutputted by the interface 16, for example.

The optical characteristics of the imaging units (10, 12) may includeany of the following: the aperture of one or both imaging units (10,12),the effective or actual focal length of one or both imaging units(10,12), lens parameters of one or both imaging units (10,12), thesensitivity to luminance and frequency response of one or both imagingunits or their components (e.g., charge coupled devices (CCD) or otherdetectors), resolution of one or both imaging units (10, 12) (e.g.,expressed as number of pixels per image or pixel dimensions) andfiltering of the output of one or both imaging units (10,12) for noise,optical distortion, chrominance distortion, luminance distortion, or forlighting variations (e.g., shade versus sunlight). The frequencyresponse of one or both imaging units (10, 12) may support thecollection of images in the following frequency spectrum of theelectromagnetic spectrum: humanly visible light, a portion of thevisible light spectrum (e.g., red light spectrum, green light spectrumand blue light spectrum or other color portions of the humanly visiblelight spectrum), near-infrared frequency range, the infrared frequencyrange, a thermal frequency spectrum, and any combination of theforegoing frequency ranges.

The left imaging unit 10 and right imaging unit 12 gather a pair of rawstereo scene images of the scene or operator, or a portion thereof,within the scene from spatially offset perspectives at a common time.The imaging units (10,12) may be mounted within a cab, cabin, bridge, orcockpit of a vehicle or externally such that an operator, or a portionthereof, within the cab, cabin, bridge, or cockpit is visible, or withinthe field of view of the imaging units (10,12). The portion of theoperator that is captured or visible in the collected image data orstereo image data may include any of the following: the operator's head,a profile of the operator's head, the operator's face, a profile of theoperator's face, the operator's head, the operator's neck, theoperator's bust, the operator's facial features, the operator's hair,the operator's forehead, and the operator's chin.

The left imaging unit 10 and the right imaging unit 12 are offset by afixed, known spatial amount, which may be referred to as the stereobaseline. The image combiner 14 combines the raw stereo scene images toproduce composite image data or a composite data stream. For example,the composite data stream or stereo vision data may consist ofalternating frames, alternating lines of frames, alternating words,bytes, bits, or data packets of left image data from the left imagingunit 10 and right image data from the right imaging unit 12, in oneembodiment, the image combiner 14 comprises an interlace that accepts aninput of a left image data stream and a right image data stream andprovides an output of an interlaced data stream that contains bytes fromboth the left image data stream and the right image data stream. Theimage collection system 15 supports synchronization of images collectedfrom the left imaging unit 10 and the right imaging unit 12 such thatimages captured at the substantially the same time are associated withone another. The image collection system 15 and/or the system 11 is wellsuited for mounting in any location (e.g., an inconspicuous location)within the cab or cockpit of a vehicle, and need not be mounted inalignment with the eyes of the operator to capture the status of theoperator's eyes, unless eye monitoring for alertness is desired.

Image Processor

The image processor 20 comprises an image segmenter 29, an imagedisparity module 22, a three dimensional image former 23, an objectposition estimator 25, a pattern recognition module 21, and acommunications interface 30. In one embodiment, the image segmenter 29communicates with the image disparity module 22. The image disparitymodule 22 communicates with the three dimensional image former 23. Inturn, the three-dimensional image former 23 communicates with the objectposition estimator 25, the pattern recognition module, 21, or both. Thepattern recognition module 21 communicates with the communicationsinterface 30. The communications interface 30 is an intermediary thatmanages communications between the image processor 20 and the analyzer32.

In one embodiment, the image processor 20 facilitates the determinationof a range of the operator, or portion thereof, with respect to theimage collection system 15 and the dimensions of an operator, or portionthereof. The image processor 20 is well suited for creating a threedimensional representation of the operator, or portion thereof, or ascene based on the disparity map image and the stereo vision data.However, to conserve data processing and computational resources, theimage segmenter 29 may extract or segment operator-related image datafrom the scene image data or background image data (e.g., cockpit, cabinor cab image data) The image segmenter 29 provides the extractedoperator-related image data to the image disparity module 22.Operator-related image data may refer to image dais that is a volume orregion of the scene that is likely to contain the operator, or a portionthereof, of image data that meets a test (e.g., color similarity ordifferentiation) that is probative of whether the image data is relatedto the operator. If the image segmenter 29 overinclusively extractsoperator-related image data that includes background data, additionaldata processing resources of the system (11 or 111) may be used.

The image disparity module 22 creates a disparity map image thatrepresents disparity between the raw stereo scene images from the leftimaging unit 10 and the right imaging unit 12. The disparity map may bebased on raw stereo images, operator-related image data, or both. Thethree-dimensional image former 23 accepts an input of a disparity imageor other stereo vision image data (e.g., interlaced data stream) of ascene (or operator, or portion thereof) and produces a three-dimensionalrepresentation of the scene (or operator, or portion thereof) as anoutput.

In one embodiment, the three dimensional image former 23 may create orform a three-dimensional image representation of the operator, or aportion thereof, based on one or more of the following types of inputdata: the disparity image, raw left image data, and raw right imagedata, stereo vision data, and interlaced image data. Thethree-dimensional representation may comprise a constellation of datapoints that lie on the surface of the operator, or a portion thereof; aframework of planar patches or a grid that lie on the surface of theoperator, or a portion thereof; a rough block or cubic representation ofthe dimensions of the operator, a portion thereof; or anotherthree-dimensional model of the operator. The data points on theoperator, the framework that models the operator, or anotherthree-dimensional model may be referenced to a stationary observationpoint of the image collection system 15 or otherwise.

The object position estimator 25 may determine or estimate one or moreof the following: (1) the range (depth 50) of the operator, or a portionthereof, from the Image collection system 15 or a reference pointassociated therewith, (2) the three-dimensional dimensions of theoperator, a portion thereof, (e.g., width, depth and height), (3)two-dimensional dimensions of the operator, or a portion thereof, (e.g.,width and length) and range to the operator; (4) an estimated center ofmass or geometric center of the operator, or a portion thereof; (5) areference axis associated with the operator's head, where the referenceaxis is defined by at least two reference points lying on or within theoperator's head, and their respective three dimensional coordinates whenthe operator is in an alert state, (6) a reference axis associated withthe operator's head, where the reference axis is defined by a firstreference point associated with the operator's forehead and a secondreference point associated with the operator's chin when the operator isin an alert stats, (7) an observed axis associated with the operator'shead, where the observed axis is defined by at least two referencepoints lying on or within the operator's head, and their respectivethree dimensional coordinates, when the operator is in any state (alertor not), (8) an observed axis associated with the operator's head, wherethe reference axis is defined by a first reference point associated withthe operator's forehead and a second reference point associated with theoperator's chin when the operator is any state (alert or not), and (9)any angular displacement or tilt between a reference axis and anobserved axis.

The range of the object may represent the distance between a surfacepoint on the operator and a reference point associated with the imagingcollection system 15 (e.g., a baseline of the imaging units (10, 12)).In an alternative embodiment, the range of the operator may representthe distance between a geometric center or center of mass of theoperator and the imaging collection system 15 or a reference pointassociated therewith. In yet another alternate embodiment, the range ofthe operator may represent the distance between a reference pointassociated with the imaging collection system 15 and some referencepoint that lies on or within the operator.

The pattern recognition module 21 or object detection module 28communicates with one or more of the following components: the objectposition estimator 25 and the three dimensional image former 23. Thepattern recognition module 21 comprises an object detection module 26,which facilitates identification of an operator, or a portion thereof,within a scene, collected image data or collected stereo vision data.The pattern recognition module 21 may facilitate the identification ofan operator, or portion thereof, in a scene from stereo image data andthe extraction of an operator-related image data from background imagedata within the Stereo vision data based on one or more of thefollowing: (1) color information (e.g., pixels or voxels) associatedwith the image, (2) luminance information (e.g., pixels or voxels),associated with the image, (3) three-dimensional shape data on theoperator, or any portion thereof, and (3) dimensions of the operator, orany portion thereof. For example, an operator, or portion thereof, mayhave a generally uniform color (e.g., skin color, flesh color, facialfeature colors, hair color, hat color, eyeglass frame color, clothingcolor) which is distinct from the background data or a particularluminance value of pixels that is distinct from the background data. Avoxel refers to a volume pixel or pixel of a three-dimensional image.

In one configuration, the object detection module 26 may differentiatean operator, or portion thereof, from the background or remainder of animage by one or more of the following techniques: (1) identifyingoperator-related image portions by matching color parameters of pixels(or voxels) in an image or portion of an image to reference colorparameters of pixels (or voxels) within one or more depth zones or athree-dimensional image, where the reference color parameters representexpected color parameters for the operator (e.g., skin color, haircolor, facial features of the operator), (2) identifyingoperator-related image portions by matching observed size parameters toa reference object profile size (e.g., typical dimensions of operators,heads, necks, busts, and facial features) within one or more depth zonesof a three-dimensional image, and (3) rejecting or filtering put imageportions associated with color parameters or pixels that are consistentwith reference color parameters of an interior of a cab, cabin, orcockpit of a vehicle. The reference color parameters of each particularoperator may be obtained from the image collection system 15 operatingunder defined lighting and head position conditions, whereas thereference color parameters of the cab, cabin or cockpit may be obtainedfrom the image collection system 15 when the cab, cabin or cock pit isempty, or otherwise,

The communications interface 30 communicates pattern recognition data,row location data, operator location data, operator avoidance data,operator dimension data, and operator range data to an analyzer 32.

Analyzer

The analyzer 32 comprises a data storage device 31 and an evaluator 36.The data storage device 31 stores reference position data 33 andreference motion data 34. The reference position data 33 may compriseposition or three dimensional coordinates, of one or more referencepoints associated with an operator, or a portion (e.g., operator's head,neck, face, or bust) thereof. For example, the reference position datamay represent a reference axis defined by two reference points lying ona surface of the operator's head, and their respective three dimensionalcoordinates, when the operator is in an alert state. The referencemotion data may comprise a change in position versus time of one or morereference points. Each reference point may be expressed in threedimensional coordinates, or otherwise consistent with the referenceposition data.

The observed position data may comprise position or three dimensionalcoordinates of one or more reference points associated with an operator,or a portion (e.g., operator's head, neck, face, or bust) thereof. Forexample, the observed position data may represent a reference axisdefined by two reference points lying on a surface of the operator'shead, and their respective three dimensional coordinates, when theoperator is in an alert state. The observed motion data may comprise achange in position versus time of one or more reference points. Eachreference point may be expressed in three dimensional coordinates, orotherwise consistent with the observed position data.

Alert Device

The alert device 40 may comprise an alarm, a siren, a buzzer, an audibleoscillator, a flashing light, a light, a light-emitting diode, adisplay, a liquid crystal display, a switch, relay or semiconductor thattriggers an electronic device (e.g., a radio or a volume control), avibrating device, a piezoelectric-transducer, or another device toalert, stimulate, or wake the operator, to prevent drowsiness, or tootherwise discourage instinctiveness. For instance, a vibrating deviceor piezoelectric transducer may be associated with the steering wheel orthe operator's seat to vibrate the steering wheel or the operator's seatin response to a triggering signal or alert signal from the analyzersystem 32.

The system of FIG. 1B is similar to the system of FIG. 1A, except thesystem of FIG. 1B further comprises a motion position estimator 27.

The motion position estimator 27 comprises the combination of an objectposition estimator 25 and a clock 31 (e.g., or timer) to determine orestimate one or more of the following: (1) whether the operator, orportion thereof, is stationary or moving (e.g., rotating or tilting),(2) whether the reference axis or the observed axis (associated with theoperator) is stationary or moving (e.g., rotating or tilting), (3)whether one or more reference points associated with the operator, theoperator's head, the operator's neck or otherwise is stationary ormoving, (4) the velocity, acceleration, speed, or heading of theoperator, or any portion thereof, relative to the imaging collectionsystem 15 or another stationary reference point, (5) the velocity,acceleration, speed, angular velocity, angular acceleration, rotationaldisplacement, or heading of reference points, a reference axis, or anobserved axis relative to the imaging collection system 15 or anotherstationary reference point and (6) any motion data related to theposition data provided by the object position estimator 25. For theconfiguration of FIG. 1B, the pattern recognition module 21 or objectdetection module 26 communicates with one or more of the followingcomponents: the object position estimator 25, the object motionestimator 27, and the three dimensional image former 23,

FIG. 2 generally describes a method for defecting operator alertness orinattentiveness for an operator of a vehicle or machine. The method ofFIG. 2 begins in step S200.

In step S200, the image collection system 15 gathers (e.g.,simultaneously gathers) a pair of raw stereo scene images of theoperator, or a portion thereof, from spatially offset perspectives. Thespatially offset perspective is preferably one in which the focal axisof the first imaging unit. 10 and the second imaging unit 12 areseparated by a known distance and orientation (e.g., fixed lineardistance called stereo baseline b in FIG. 3).

In step S201, an image processor 20 or image segmented 29 identifies anoperator related image data in the gathered stereo scene images. Forexample, the image processor 20 or the image segmenter 29 identifiesoperator-related image data by analyzing one or more images in colorspace. The image processor 20 or the image segmenter 29 may identify,extract, distinguish or discern the operator-related image data from thebackground data of the image data in color space by applying one or moreof the following techniques to the image or a portion of an image: colordifferentiation, color histogram analysis, probability density functionanalysis, and edge analysis in an image region within the boundaries ofthe operator and immediately outside the boundaries of the operator, inone embodiment, to differentiate the operator from the background of theimage or scene (e.g., interior of the cab or cockpit), the imagesegmenter 29 or image processor 20 may apply data processing resourcesof the image processor 20 first to a candidate region of the scene wherethe operator is supposed to be seated and secondarily to the entirefield of view of the image collection system 15.

To accordance with a first technique for executing step S201, the imageprocessor 20 or the image segmenter 29 compares a collected derivativedata set of the collected image to a reference derivative data set of areference image. In one embodiment, the collected derivative data setmay represent a probability density function or a color histogramanalysis applied to a collected image or a portion thereof. Further, thereference derivative data set of a reference image may be defined as aprobability density function of color parameters (e.g., color, tone,hue, or intensity) or color histogram of color parameters derived fromone or more reference images (e.g., of the operator in interior of thecockpit or cab of the vehicle). The reference image may be taken atregular intervals or each time a vehicle is started or used, forexample.

In accordance with a second technique for executing step S201, the imageprocessor 20 or the image segmenter 29 identifies the presence of anoperator, or portion thereof, in (pixels or voxels of) the collectedimage with one or more colors or color attributes (e.g., flesh tone,skin tones or hair color) that are sufficiently distinct to a particularcorresponding operator. Where a portion of the (pixels or voxels) of thecollected image has colors substantially match or are sufficientlycorrelated to the reference image, the operator, or a portion thereof,is identified as present. In one embodiment, the image processor 20 orobject detection module 25 identifies the outline, object boundary, oredges of the object or operator by distinguishing the operator color ofthe operator from the background color of a background image data ordistinguishing a material color transition between the boundary of theoperator and the background through comparisons of groups of pixels orvoxels in the region of the image.

Advantageously, in step S201 the application of color differentiation,color probability density function, histogram and edge analysis may becompleted rapidly to identify operators, or portions thereof, in astructured environment, such as within a cab or cockpit of a vehicle.

In step S202, an image disparity module 22 or image processor 20combines raw stereo scene images or operator-related image data toproduce a disparity map that represents disparity between the raw stereoscene images collected in step S200. The raw stereo images may beexpressed as or converted into greyscale images as inputs to thedisparity module 22. The disparity module 22 uses a disparity algorithm(e.g., correlation of common points in each raw stereo image or a sum ofabsolute differences applied to such common points) to form a disparitymap based on a spatial relationship between each point in one raw stereoimage its corresponding point in the other stereo image. The disparitymodule creates a disparity map for each pixel in one of the raw stereoscene images. The disparity map may be stored as a single greyscaleimage or a monochrome image in which disparity is a function of x and yin one raw stereo scene image or d (x,y), for example.

Stereo disparity (d) is inversely proportional to the depth of anoperator. The stereo disparity (d) is defined as d=bf/z, where b is thestereo baseline, f is the focal length of the imaging units (10 and 12),and z is the depth of an operator from the imaging units (10 and 12).The disparity map is a representation that facilitates the removal ofsome redundant data from the two raw stereo images collected in stepS200.

In step S204, the three dimensional image former 23 or the imageprocessor 20 creates a three-dimensional representation of the scene,the operator, or a portion thereof, based on the disparity map image andat least one of the raw stereo scene images gathered in step S200. Inone example for carrying out step S204, the three dimensional imageformer 23 may accept an input of the disparity map, which hasinformation about x,y,d for the scene or an operator, and map the x,y,dinformation to x,y,z coordinates for the scene or any operator in thescene.

In another example for carrying out step S204, the three dimensionalimage former 23 extracts the three-dimensional map from the disparitymap and at least one of the raw stereo scene images through (a) theapplication of the stereo vision equations discussed in more detail inFIG. 3 to estimate the three-dimensional real world coordinates of theoperator from the perspective of the imaging system and (b) constructionof a surface model of the operator based on the estimate of threedimensional world coordinates.

The construction of the surface model may use a grid of voxels thatgenerally defines the three-dimensional shape of an operator to form orinterconnect the nearest neighbor points with linear segments or tangentplanar grids, for example. Although greater resolution of the surfacemodel of the operator is useful for operator classification or theapplication of pattern recognition techniques, the surface model can becrude to produce a rapid surface solution for detecting the position ormotion of one or more of the following: an operator, an operator's head,an operators neck, an operator's bust, any reference points lying withinor one surface associated with the operator, and any reference pointslying within or on any portion of the operator,

In step S206, an object position estimator 25 or image processor 20estimates three dimensional coordinates associated with one or morepoints of the three dimensional representation. For example, the depth(d) of the operator, or a portion thereof, is estimated by correlationof common data points in the limited region of the operator in a leftimage and a corresponding right image of a stereo image. Once the depthis available, real world y data within the limited region of theoutline, boundary or edges of the operator, or a portion thereof, may becomputed by reference to either the right image or the left image. Byprocessing only a limited region within the outline, boundary or edgesof the operator, the data processing resources are minimized and timereduced to practice the method of FIG. 2.

In step S208, to reduce computational time and data processingresources, the image processor 20 or object defection module 26 limitsthe analysis or image processing to any of the following: (1) aparticular region of depth of a three-dimensional image, (2) a twodimensional depth slice of uniform depth of three-dimensional image, (3)an image (e.g., a two-dimensional image) from one of the imaging units(19 or 12).

In to step S207, the image collection system 15 collects one or morereference images (e.g., stereo vision data) of the operator, anoperator's head region, another portion of the operator. The imageprocessor 20 uses the reference image or images to establish a referenceposition of one or more reference points of the operator or a portionthereof to indicate the operator is in an alert state. The referenceimage may be taken when the operator is generally alert or attentive,for example.

Steps S200, S201, S202, S204, and S208 shall be collectively known asstep S199, which relates to a stereo vision imaging process. During theprevious execution of steps S200, S201, S202, S204, and S206, the stepswere executed in preparation for establishing the reference position instep S207. After step S207, step S199 or the stereo vision imagingprocess is executed in preparation for determining a change in observedposition or observed motion in step S208.

In step S208, the analyzer 32 or image processor 20 determines a changeIn an observed position of the reference points or a change in observedmotion of the reference points that indicates an operator is not alertor attentive. For example, the reference points of the operator, or aportion thereof, may shift from the established reference position to anobserved reference position that indicates that the operator isinattentive, asleep, unconscious, or otherwise not alert.

FIG. 3 is an illustration of a typical representation of howthree-dimensional information on an object or operator representation isextracted from an image collection system 15 that collects stereo visionimages. An operator, or a portion thereof, may be regarded as an objectin the collected image data, stereo vision data, operator-related imagedata, or the like. The operator-related image data may relate to ordefine one or more of the following: an operator, an operator's head, anoperator's face or portion thereof, an operator's nose, an operator'sforehead, an operator's chin, an operator's mouth, a operator's facialfeature, an operator's neck, an operator's hair or hat, an operator'sclothing, and an operator's bust.

A right lens center 45 and a left lens center 44 are associated with theright imaging unit 12 and the left imaging unit 10, respectively. Theright lens center 45 and left lens center 44 may represent centralregions of the lenses of the imaging units. A dashed reference line,labeled “b” for stereo baseline b, interconnects the right lens center45 and the left lens center 44. The first imaging unit 10 and the secondimaging unit 12 are separated by a distance associated with the stereobaseline (b). The optical axes of each imaging unit is perpendicular tothe stereo base Sine. An operator head 48 is separated from baseline bby a depth 50. A group of points on the operator head 48 may beexpressed as three dimensional information that defines the shape, size,and spatial dimensions of the operator head 48.

The three-dimensional information may be represented in a Cartesiancoordinate system, a polar coordinate system, a spherical coordinatesystem, or otherwise. As illustrated in FIG. 3, the three dimensionalinformation is expressed in accordance with Cartesian coordinates. Forexample, the three dimensional information may include coordinate values(e.g., x, y, z) for each point on the surface of the operator head 48.As illustrated in FIG. 3, the three dimensional information isreferenced to an x-y image plane generally parallel to base line b ofthe imaging units (10 and 12). A z-axis is perpendicular to or normal tothe x-y image plane.

The left image 40 and the right image 42 lie in the image plane (i.e.,the x-y plane) in FIG. 3. The left image 40 represents a view of theoperator head 48 (and surrounding image) from the perspective of theleft imaging unit 10, which includes the left lens center 44. The rightimage 42 represents a view of the operator representation 48 (andsurrounding image) from the perspective of the right imaging unit 12,which includes the right lens center 45. The right image 42 lies behindthe right lens center 45 by a focal length, designed f_(r); the leftimage 40 lies behind the left lens center 44 by a focal lengthdesignated f_(l). A left image coordinate system is defined on the leftimage plane as x_(l), y_(l). A right image coordinate system is definedon the right image plane, as x_(r), y_(l).

The three dimensional world coordinates of a point in a scene or on anobject (e.g., operator or head) in the scene are as follows:

${x = \frac{b\left( {x_{1}^{\prime} - x_{f}^{\prime}} \right)}{2d}},{y = \frac{\left( {y_{1}^{\prime} - y_{r}^{\prime}} \right)}{2d}},{and}$$z = \frac{bf}{d}$

where b is the stereo baseline distance between the optical centers ofthe two imaging units, d is the disparity, which is x′_(l)-x′_(f) and fis the effective focal length of the imaging units for the case wheref=f_(f), f_(l), x′_(l) is the x_(l) coordinate on the left image of theimage plane corresponding to the point on the object, y′_(l) is y_(l)coordinate on the coordinate on the left image plane, x′_(r) is thex_(r) coordinate on the right image of the image plane corresponding tothe point on the object, and y′_(r) is y_(r) coordinate on thecoordinate on the right image plane. The z dimension is synonymous withthe depth of a point on the object.

The above equations may be applied repeatedly (e.g., periodically or atregular intervals or sampling times) during operation of the vehicle. Asthe operator alertness system (11 or 111) moves throughout a work areaor its environment to capture a group of stereo scene images of anoperator head 48. Periodically or at regular time intervals, additionalstereo images may be collected to extract further three-dimensionalinformation on the same operator, or portion thereof, to detect anychanges over time that may indicate an operator's level of alertness.Accordingly, a three-dimensional reconstruction of the dimensions andrange of operator, or portion thereof, may be based on three dimensionalimage points calculated at different times and registered to a commonreference or coordinate system with respect on one another. Registrationof different stereo images taken at different times may be accomplishedby selecting portions of the scenes and matching for luminance, color orintensity of the pixels or voxels. Corresponding luminance values, andcolor values (e.g., in RGB color space, or otherwise) may be associatedwith each data point to form a voxel or another multidimensionalrepresentation.

FIG. 4A shows a profile of an operator or operator head 48 in a positionthat indicates an alert state of an operator. A reference axis 128 isdefined by a first reference point 131 and second reference point 133associated with a head of an operator in a position that indicates analert state. For example, the first reference point 131 may lie on aforehead or other surface of the operator head 48, whereas the secondreference point 133 may lie on a chin of the operator head 48. Thereference axis 129 may have a reference angle 127 (also referred to asθ_(R)) to a standard axis 125 of known orientation or generally verticalaxis.

FIG. 4B and FIG. AC show a profile of an operator head 48 in a positionthat indicates an inattentive state of an operator. Like referencenumbers in FIG. 4A, FIG. 4B and FIG. 4C indicate like elements.

In FIG. 4B, the operator head 48 is tilted forwards by an amount whichexceeds a displacement threshold (e.g., critical angle), which ispotentially indicative of or correlated to an operator that is not in analert state, in FIG. 48, the operator head 48 is tilted toward by anobserved angle 237 (which is also labeled θ) that exceeds a displacementthreshold (e.g., first critical angle 235, which is also labeledθ_(CF)). Because the observed angle 237 exceeds the first critical angle235, the detection system (11 or 111) indicates that an Operator that isnot in an alert state and may trigger the issuance of an alarm via alertdevice 40.

The observed angle 237 relates to the angle formed between the standardaxis 125 (e.g., generally vertical axis) and the observed axis 239. Theobserved axis 239 is defined by a first reference point 231 and a secondreference point 233. The observed first reference point 231 is on thesame position (e.g., operator's forehead) of the operator head 43 as thefirst reference point 131, but is displaced in absolute threedimensional coordinates. Similarly, the observed second reference point233 is on the same position (e.g., operator's chin) of the operator head48 as the second reference point 133, but is displaced in absolute threedimensional coordinates.

In FIG. 4C, the operator head 48 is tilted backwards by an amount whichexceeds a displacement threshold (e.g., critical angle), which ispotentially indicative of or correlated to an operator that is not in analert state. In FIG. 4C, the operator head 48 is tilted forward by anobserved angle 337 (which is also labeled θ) that exceeds a displacementthreshold (e.g., second critical angle 335, which is also labeledθ_(CB)). Because the observed angle 337 exceeds the second criticalangle 335, the detection system (11 or 111) indicates that an operatorthat is not in an alert state and may trigger the issuance of an alarmvia alert device 40.

The observed angle 337 relates to the angle formed between the standardaxis 125 (e.g., generally vertical axis) and the observed axis 339. Theobserved axis 339 is defined by a first reference point 331 and a secondreference point 333. The observed first reference point 331 is on thesame position (e.g., operator's forehead) of the operator head 48 as thefirst reference point 131, but is displaced in absolute threedimensional coordinates. Similarly, the observed, second reference point331 is on the same position (e.g., operator's chin) of the operator head48 as the second reference point 333, but is displaced in absolute threedimensional coordinates.

FIG. 5 is a flow chart of a method for determining an alertness of anoperator of a vehicle or machine. The method of FIG. 5 begins in stepS400.

In step S400, an image collection system 15 or an imaging systemcollects reference position data (e.g., reference head position data)associated with an operator, or a portion thereof, (e.g., an operator'shead) when the operator is in an alert state An alert state refers toone in which the operator is conscious, awake, mentally aware, and/orattentive of the operation and control of the vehicle or machine.

The reference position data may be defined in accordance with variousdefinitions that may be applied alternately or cumulatively. Under afirst definition, the reference position data comprises the threedimensional coordinates (e.g., Cartesian or polar coordinates)associated with an operator, or a portion thereof. Under a seconddefinition, the reference position data comprises the three dimensionalcoordinates associated with one or more reference points (e.g., firstreference point (131) and a second reference point (133) of FIG. 4A) ofthe operator head 48. Under a third definition, the reference positiondata comprises a reference axis defined by two reference points lying ona surface of the operator's head, and their respective three dimensionalcoordinates, when the operator is in an alert state.

In one embodiment, step S400 may be carried by executing the stereovision process of step S199 (which was previously described inconjunction with FIG. 2) to create a three dimensional representation ofthe operator when the operator is in an alert state and selecting one ormore reference points associated with the three dimensionalrepresentation.

In step S402, the image processor 20 or object position estimator 25determines observed position data (e.g., observed head position data) ofone or more reference points of a three dimensional representation ofthe operator during a time interval. The observed position data may bedefined in accordance with various definitions that may be appliedalternately or cumulatively. Under a first definition, the observed,position data comprises three dimensional coordinates (e.g., Cartesianor polar coordinates) of one or more reference points associated with anoperator, or a portion thereof. Under a second definition, the observedposition data may comprise the three dimensional coordinates associatedwith one or more reference points (e.g., first reference point (231 or331) and a second reference point (233 or 333) of FIG. 4B or FIG. 4C) ofthe operator head 48. Under a third definition, the observed positiondata comprises a reference axis defined by two reference points lying ona surface of the operator's head, and their respective three dimensionalcoordinates.

In one embodiment, step S402 may be carried by executing the stereovision process of step S199 to create a three dimensional representationof the operator when the operator is under observation to monitor theoperator's alertness. Further, the image processor 20 or image positionestimator 25 selects one or more reference points associated with thethree dimensional representation as the observed position data.

In step S404, the analyzer 32 or evaluator 36 determines whether adetected angular shift (e.g., an inclination, forwards, backwards, orsideways from a generally vertical axis) of one or more of points of therepresentation exceed s displacement threshold. For example, theanalyzer 32 or evaluator 36 determines whether a defected angular shift(e.g., an inclination, forwards, backwards, or sideways from a generallyvertical axis) from the reference position data to the observed positiondata of one or more of points of the representation exceed adisplacement threshold. A displacement threshold refers to a distancethat exceeds a minimum distance or an angle that exceeds a criticalangle (e.g., first critical angle 235 of FIG. 4B or second criticalangle 335 of FIG. 4C) or minimum angle. For example, where the referencepoints define an observed axis, the detected angular shift may representan inclination forwards, backwards or sideways of the observed axis withrespect to a generally vertical axis by at least a critical angle.

If the analyzer 32 or evaluator 36 determines that the detected angularshift exceeds a displacement threshold, the method continues with stepS406. However, if the analyzer 32 or evaluator 36 determines that thedetected angular shift does not exceed a displacement threshold, themethod continues with step S408.

In step S406, an analyzer 32 or an alert device 40 sends an alert signalto alert an operator. For example, the alert device 40 generates anaudible tone or alarm to wake or otherwise alert the operator. Afterstep S406, the method may continue with step S408, for example.

In step S408, the image collection system 15 waits a time interval(e.g., a sampling interval) prior to returning to step S400 to collectreference position data.

FIG. 6 is a flow chart of a method for determining an alertness of anoperator. The method of FIG. 6 begins in step S500.

In step S500, an image collection system 15 or an imaging systemcollects reference motion data (e.g., reference head motion data)associated with an operator, or a portion thereof, when the operator isin an alert state. For example, the image collection system 15 or theimaging system collects motion data as at least two three dimensionalrepresentations of an operator, or a portion thereof, over a first timeperiod via stereo vision processing.

The reference motion data may be defined in accordance with variousdefinitions that may be applied alternately or cumulatively. Under afirst definition, the reference motion data may comprise movementassociated with the three dimensional coordinates of one or morereference points associated with an operator, or a portion thereof.Under a second definition, the reference motion data comprises movementassociated with a reference axis defined by two reference points lyingon a surface of the operator's head, and their respective threedimensional coordinates, when the operator is in an alert state.

In one embodiment, step S500 may be carried by executing the stereovision process of step S199 (which was previously described inconjunction with FIG. 2) to create multiple three dimensionalrepresentations of the operator at corresponding times when the operatoris in an alert state and selecting one or more reference pointsassociated with the three dimensional representation. The clock 31 orobject motion estimator 31 may track the elapsed time (e.g., first timeperiod) between the multiple three dimensional images to derivereference motion data characteristic of an operator in an alert state.

In step S502, the image processor 20 or object position estimator 25determines observed motion data (e.g., observed head motion data) of oneor more reference points of a three dimensional representation of theoperator during a time interval. For example, the image processor 20 orobject position estimator 25 determines observed motion data as at leasttwo three dimensional representations of an operator, or a portionthereof, over a second time period via stereo vision processing.

The observed motion data may be defined in accordance with variousdefinitions that may be applied alternately or cumulatively. Under afirst definition, the observed motion data comprises movement associatedwith three dimensional coordinates of one or more reference pointsassociated with an operator, or a portion thereof. Under a seconddefinition, the observed motion data comprises movement associated witha reference axis defined by two reference points lying on a surface ofthe operators head, and their respective three dimensional coordinates,

In one embodiment, step S502 may be carried by executing the stereovision process of step S198 (which was previously described inconjunction with FIG. 2) to create multiple three dimensionalrepresentations of the operator at corresponding times when the operatoris operating the vehicle and selecting one or more reference pointsassociated with the three dimensional representation. The clock 31 orobject motion estimator 31 may track the elapsed time (e.g., second timeperiod) between the multiple three dimensional images to derive observedmotion data of the operator while operating the vehicle.

In step S504, the analyzer 32 or evaluator 36 determines whether adetected angular shift (e.g., an inclination, forwards, backwards, orsideways from a generally vertical axis) per unit time of one or morereference points of the representation exceeds a motion threshold. Forexample, the analyzer 32 or evaluator 36 determines whether a detectedangular shift (e.g., an inclination, forwards, backwards, or sidewaysfrom a generally vertical axis) from the reference motion data to theobserved motion data of one or more of points of the representationexceed a motion threshold. A motion threshold refers to a distance thatexceeds a minimum distance or an angle that exceeds a minimum orcritical angle per unit time or over an elapsed time period of knownduration. For example, where the reference points define an observedaxis, the detected angular shift may represent an inclination forwards,backwards or sideways of the observed axis with respect to a generallyvertical axis by at least a critical angle over an elapsed time period.

In an alternate embodiment, the motion threshold may be defined as alack of motion over an elapsed time period (or minimal displacement overan elapsed time period) that is correlated to or indicative of anoperator in a potentially inattentive state or degraded alertness.

If the analyzer 32 or evaluator 36 determines that the detected angularshift exceeds a motion threshold, the method continues with step S406.However, if the analyzer 32 or evaluator 36 determines that the detectedangular shift does not exceed a motion threshold, the method continueswith step S408.

In step S408, the image collection system 15 waits a time interval(e.g., a sampling interval) prior to returning to step S500 to collectreference motion data. After step S408, the method may continue withstep S408, for example.

FIG. 7 is a flow chart of a method for determining an alertness of anoperator. Like reference numbers in FIG. 7 and FIG. 5, or in FIG. 7 andFIG. 8, indicate like steps or procedures. The method of FIG. 7 beginsin step S400.

In step 3400, an image collection system 15 or an imaging systemcollects reference position data (e.g., reference head position data)associated with an operator, or portion thereof, (e.g., an operator'shead) of a vehicle when the operator is in an alert state.

In step S500, an image collection system 15 or an imaging systemcollects reference motion data (e.g., reference head motion data)associated with an operator, or portion thereof, (e.g., an operator'shead) when the operator is in an alert state.

In step S402, the image processor 20 or object position estimator 25determines observed position data (e.g., observed head position data) ofone or more reference, points of a three dimensional representation ofthe operator during a time interval.

In step S502, the image processor 20 or object position estimator 25determines observed motion data (e.g., observed head motion data) of oneor more reference points of a three dimensional representation of theoperator during a time interval.

in step S404, the analyzer 32 or evaluator 36 determines whether adetected angular shift (e.g., an inclination, forwards, backwards, orsideways from a generally vertical axis) of one or more of points of therepresentation exceed a displacement threshold. A displacement thresholdrefers to a distance that exceeds a minimum distance or an angle thatexceeds a critical angle or minimum angle. If the analyzer 32 orevaluator 36 determines that the detected angular shift exceeds, adisplacement threshold, the method continues with step S406. However, ifthe analyzer 32 or evaluator 36 determines that the detected angularshift does not exceed a displacement threshold, the method continueswith step S504.

In step S406, an analyzer 32 or an alert device 40 sends an alert signalto alert an operator. For example, the alert device 40 generates anaudible tone or alarm to wake or otherwise alert the operator.

In step S504, the analyzer 32 or evaluator 36 determines whether adetected angular shift (e.g., an inclination, forwards, backwards, orsideways from a generally vertical axis) of one or more of points of therepresentation exceed a motion threshold. A motion threshold refers tomovement exceeding a minimum distance within a time window, or movementexceeding an angular displacement within a time window. For example, theangular displacement means a critical angle (e.g., first critical angle235 of FIG. 4B or second critical angle 335 of FIG. 4C) or minimumangle. If the analyzer 32 or evaluator 36 determines that the detectedangular shift exceeds a motion threshold, the method continues with stepS406. However, if the analyzer 32 or evaluator 36 determines that thedetected angular shift does not exceed a motion threshold, the methodcontinues with step S408.

In step S408, the image collection system 15 waits a time interval(e.g., a sampling interval) prior to returning to step S400 to collectreference position data.

FIG. 8 is a flow chart of another method for determining an alertness ofan operator.

In step S700, an image collection system 15 determines if at least oneeye of an operator is visible, if at least one eye of the operator isvisible, the method continues with step 3702. However, if no eyes of theoperator are visible, the method continues with step S708.

In Step S702, the image collection system 15 determines whether or notthe operator is generally facing forward (e.g., in a cab or cockpit ofthe vehicle). Here, forward refers to the toward direction of travel ofthe vehicle or the front of the vehicle. The operator may be regarded asfacing forward if the operator is within a certain lateral angular range(e.g., plus or minus 40 degrees) of absolute forward or the forwarddirection of travel of the vehicle. If the operator is generally facingforward, the method continues with step S704. However, if the operatoris not facing forward, the method continues with step S708.

In step S704, the eye monitoring algorithm and/or equipment detectsalertness of an operator. The eye monitoring algorithm and/or equipmentmay determine one or more of the following: (1) whether one or both eyesof the operator are open or closed, (2) the rate at which the operatorblinks, (3) the ratio or percentage of eye closure (PERGLOS) over a timeperiod, and (4) whether the movement of the pupil of the operator's eyeis consistent with alertness. For example, whether or not one or both ofthe operator eyes are open may be determined by the image processor 20applying color differentiation (e.g., open eye colors versus closed eyecolors) in the region of the operator's eye. The use of such eye relateddata may require filtering to remove the effects of eye movementsassociated with cognitive demands of the task and brightness of theambient lighting that might otherwise be perceived as an indicator ofalertness or inattentiveness of the operator. Any commercially availableeye monitoring algorithms and/or equipment may be used to execute stepS704.

In step S706, which may be carried out after step S700 or S702, theimaging system or image processor 20 applies position analysis, motionanalysis, or both to a three dimensional representation of the operator(e.g., the operator's head, head and neck region, or bust) to determinealertness of the operator. Step S706 may be carried out in accordancewith various methods, which may be applied individually,, cumulatively,or collectively. Under a first method, the method if FIG. 5 may beapplied to carry out step S706. Under a second method, the method ofFIG. 6 may be applied to carry out step S706. Under a third method, themethod of FIG. 7 may be applied to carry out step S706.

Under a fourth method, the image collection system 15 or the system (11or 111) collects reference position data associated with an operator, ora portion thereof, when the operator is in an alert state; the imageprocessor 20 or analyzer system 32 determines observed position data ofone or more points of a three dimensional representation of the operatorduring a time interval; and an alert device 40 or analyzer system 32sending an alert signal to alert the operator if a detected angularshift of one or more points of the representation exceed a displacementthreshold. Pursuant to the fourth method, the image processor 20 or thethree dimensional image former 23 establishes a three dimensionalrepresentation of an operator via stereo vision processing, which maycomprise gathering a pair of raw stereo scene images of the operator, ora portion thereof, from spatially offset perspectives; combining the rawstereo scene images to produce a disparity map image that represents adisparity between the raw stereo scene images; and creating a threedimensional representation of the operator, or a portion thereof, basedon the disparity map and the raw stereo scene images.

Under a fifth method, the image collection system 15 or the system (11or 111) collects reference motion data associated with an operator, or aportion thereof, when the operator is in an alert state; the imageprocessor 20 or analyzer system 32 determines observed motion data ofone or more points of a three dimensional representation of the operatorduring a time interval; and an alert device 40 or analyzer system 32sending an alert signal to alert the operator if a detected angularshift of one or more points of the representation exceed a motionthreshold. Pursuant to the fifth method, the image processor 20 or thethree dimensional image former 23 establishes multiple three dimensionalrepresentations of an operator via stereo vision processing, which maycomprise gathering a pair of raw stereo scene images of the operator, ora portion thereof, from spatially offset perspectives; combining the rawstereo scene images to produce a disparity map image that represents adisparity between the raw stereo scene images; and creating one or morethree dimensional representations of the operator, or a portion thereof,based on the disparity map and the raw stereo scene images.

Although the operator primarily refers to a driver of a vehicle, anyembodiment of the system and method may be extended to the operator of amachine, the operator of electronic equipment, or the operator otherequipment. The system and method for detecting operator alertness iswell suited for monitoring operator alertness of operators that weareyeglasses or sunglasses. Further, the system and method for detectingoperating alertness may be applied to vehicle configurations or taskswhere the operator lends not to face forward in a vehicle at all timesduring normal operation (e.g., certain construction equipment,agricultural equipment, or mining equipment).

Having described the preferred embodiment, it will become apparent thatvarious modifications can be made without departing from the scope ofthe invention as defined in the accompanying claims.

1. A method for detecting operator alertness of an operator of a vehicleor machine, the method comprising: collecting reference position dataassociated with an operator when the operator is in an alert state;determining observed position data of one or more reference points of athree dimensional representation of the operator, or a portion thereof,during a time interval; and sending an alert signal to alert theoperator if a detected angular shift of one or more reference points,associated with an operator, exceed a displacement threshold.
 2. Themethod according to claim 1 wherein the collecting reference positiondata comprises creating a three dimensional representation of anoperator, or a portion thereof, via stereo vision processing.
 3. Themethod according to claim 1 wherein the determining observed positiondata comprises creating a three dimensional representation of anoperator, or a portion thereof, via stereo vision processing.
 4. Themethod according to claim 1 wherein the collecting reference positiondata further comprises: gathering a pair of raw stereo scene images ofthe operator, or a portion thereof, from spatially offset perspectives;identifying respective operator-related image data in the gathered rawstereo scene images; combining at least one of the raw stereo sceneimages and the operator-related image data to produce a disparity mapimage that represents a disparity between the raw stereo scene images;and creating a three dimensional representation of the operator, or aportion thereof, based on the disparity map and the raw stereo sceneimages.
 5. The method according to claim 1 wherein the determiningobserved position data further comprises: gathering a pair of raw stereoscene images of the operator, or a portion thereof, from spatiallyoffset perspectives; identifying respective operator-related image datain the gathered raw stereo scene images; combining at least one of theraw stereo scene images and the operator-related image data to produce adisparity map image that represents a disparity between the raw stereoscene images; and creating a three dimensional representation of theoperator, or a portion thereof, based on the disparity map and the rawstereo scene images.
 6. The method according to claim 1 wherein thereference position data comprises three dimensional coordinates of oneor more reference points associated with an operator, or a portionthereof.
 7. The method according to claim 1 wherein the referenceposition data comprises a reference axis defined by two reference pointslying on a surface of the operator's head, and their respective threedimensional coordinates, when the operator is in an alert state.
 8. Themethod according to claim 1 wherein the observed position data comprisesthree dimensional coordinates of one or more reference points associatedwith an operator, or a portion thereof.
 9. The method according to claim1 wherein the observed position data comprises a reference axis definedby two reference points lying on a surface of the operator's head, andtheir respective three dimensional coordinates.
 10. The method accordingto claim 1 wherein the detected angular shift represents an inclinationforwards, backwards, or sideways of one or more reference points. 11.The method according to claim 1 wherein the reference points define anobserved axis and wherein the detected angular shift represents aninclination forwards, backwards or sideways of the observed axis withrespect to a generally vertical axis by at least a critical angle.
 12. Asystem for detecting operator alertness of an operator of a vehicle ormachine, the system comprising: an image collection system forcollecting reference position data associated with an operator when theoperator is in an alert state; an image processor for determiningobserved position data of one or more reference points of a three,dimensional representation of the operator, or a portion thereof, duringa time interval; and an analyzer for sending an alert signal to alertthe operator if a detected angular shift of one or more referencepoints, associated with an operator, exceed a displacement threshold.13. The system according to claim 12 wherein the image processor furthercomprises an image former for creating a three dimensionalrepresentation of an operator, or a portion thereof.
 14. The systemaccording to claim 12 further comprising: a plurality of imaging unitsfor gathering a pair of raw stereo scene images of the operator, or aportion thereof, from spatially offset perspectives; an image segmenterfor identifying respective operator-related image data in the gatheredraw stereo scene images; an image disparity module for combining atleast one of the raw stereo scene images and the operator-related imagedata to produce a disparity map image that represents a disparitybetween the raw stereo scene images; and an image former for creating athree dimensional representation of the operator, or a portion thereof,based on the disparity map and the raw stereo scene images.
 15. Thesystem according to claim 12 wherein the reference position datacomprises three dimensional coordinates of one or more reference pointsassociated with an operator, or a portion thereof.
 16. The systemaccording to claim 12 wherein the observed position data comprises threedimensional coordinates of one or more reference points associated withan operator, or a portion thereof.
 17. The system according to claim 12wherein the reference position data comprises a reference axis definedby two reference points lying on a surface of the operator's head, andtheir respective three dimensional coordinates, when the operator is inan alert state.
 18. The system according to claim 12 wherein theobserved position data comprises three dimensional coordinates of one ormore reference points associated with an operator, or a portion thereof.19. The system according to claim 12 wherein the observed position datacomprises a reference axis defined by two reference points lying on asurface of the operator's head, and their respective three dimensionalcoordinates.
 20. The system according to claim 12 wherein the detectedangular shift represents an inclination forwards, backwards, or sidewaysof one or more reference points.
 21. The system according to claim 12wherein the reference points define an observed axis and wherein thedefected angular shift represents an inclination forwards, backwards orsideways of an observed axis with respect to a generally vertical axisby at least a critical angle.